Photon-counting based spectral CT systems for examining an object, e.g. a patient or a material, such as a tyre or cast part, require detectors, which can deal with the high count rate generated in today's energy-integrating CT systems. The commonly used direct conversion material is not fast enough to deal with the high count rates, which can occur in such systems. In particular, detector pixels behind the object close to the direct beam (“surface beams”) or directly seeing the direct beam usually see such a high count rate that they are saturated, i.e. they do not provide a usable counting signal, especially not with sufficient energy-information; for simplicity, the term “surface beams” here also comprises beams, which are so weakly attenuated that they cause pixels to see too high a count rate, although geometrically these beams are not close to the surface of the object. This can mean on the one hand that due to the very high count rate pulses can no longer be distinguished from each other, i.e. the pixels (also called “sensor elements” of the sensor included in the detector hereinafter) are saturated (such a pixel or sensor element is also called a “piled-up pixel” hereinafter). On the other hand, it can mean that due to massive charge trapping a part or all of the volume of the detector pixels become polarized, i.e. the internal electric field breaks down so that electron-hole pairs generated in the crystal due to interaction with x-ray photons are no longer efficiently separated.
In the latter situation it is difficult or even impossible to gain correct information from the measurement data, neither with a photon-counting detector nor with an energy-integrating detector, since due to the weakened electric field the majority of the electron-hole pairs generated in an x-ray interaction is not collected so that the energy information is unpredictably corrupted. However, in the first situation (i.e. case of a “saturated” or “piled-up” pixel or sensor element), a solution seems possible which is provided by the present invention.